This invention relates generally to bimetal electrodes for spark plugs, and more particularly to bimetal electrodes for spark plugs which utilize a heat resisting metal for the exposed portion and a highly thermally conductive material for the core and to novel and improved methods and apparatus for producing such electrodes.
Modern spark ignition internal combustion engines have greatly lengthened periods between service, and it has been recognized that spark plug life is an important factor in determining this interval. In such modern engines, the primary problem of spark plug life becomes that of erosion of the electrodes to a point that the gap increases beyond tolerable limits. In an effort to increase spark plug life by decreasing the erosion of the electrodes, it has been proposed to reduce the electrode temperatures by forming the electrode with an outer surface of a material such as nickel and a core of a highly thermally conductive material such as copper to rapidly conduct the heat away from the firing tip to the exposed end of the spark plug where the heat can be more easily dissipated.
Many methods have been proposed for the manufacture of such electrodes, one of which is disclosed in the application of N. I. Kin and G. T. Payne, Ser. No. 232,954, filed Feb. 9, 1981, and assigned to the assignee of the present invention. In general, this method includes the steps of forming a hollow cup from a heat-resisting metal such as nickel, and thereafter inserting in the open end of the hollow cup a cylindrical slug of a thermally conductive metal such as copper. The copper is then upset within the cup to eliminate all voids, and the resultant composite blank is extruded, closed end first, through a die to reduce the diameter to that of the finished electrode, after which further operations can be performed on the open end, where both the copper and nickel are exposed.
While this method is applicable to metals such as substantially pure nickel, it has not been used successfully with more refractory metals and alloys, which are harder to work but more desirable in a spark plug because of their increased strength and resistance to erosion at high temperatures.